1. Field of the Invention
The present invention relates generally to electronic circuits. More particularly, the present invention relates to an electronic circuit assembly, such as a printed circuit board, having reinforced solder joints.
2. Disclosure Information
In the conventional manufacture of printed circuit boards (PCBs), electronic surface mount components (SMCs) such as integrated circuit chips, chip resistors, etc. are placed on a PCB substrate with their terminations registered atop corresponding solder-pasted mounting pads, whereupon the PCB is passed through a reflow oven so that the solder paste melts, reflows, and solidifies so as to form solid solder joints which mechanically and electrically connect each SMC termination with its respective mounting pad.
However, it is often the case that the SMCs and the substrate to which they are attached have significantly different coefficients of thermal expansion (CTEs). This induces mechanical stress in the solder joints when the PCB is exposed to thermal cycling, which can eventually lead to cracks in the solder joints.
Various approaches have been proposed for alleviating this problem. For example, metal and other fillers having mechanical strength higher than the solder have been added to the solder paste, in order to make the resulting joint more mechanically robust. Also, attempts have been made to place metal foils, sheets, or screens within the solder joint interposed between the component termination and its respective mounting pad. Other approaches involve attempts at controlling the solder joint height, volume, profile, and/or other solder joint features, as well as altering the mounting pad and/or termination shapes and orientations. Additionally, materials other than solder, such as electrically conductive polymeric adhesives (e.g., silver-filled epoxy), have been used instead of traditional tin/lead solder in order to ameliorate solder joint cracking, but such alternative joint materials often present other drawbacks and limitations. Various other approaches (e.g., underfilling the SMC with an adhesive) are also known in the art for reducing the problem of thermally induced solder joint cracking.
While all of the foregoing approaches have merit in addressing this problem, it is nonetheless desirable to provide additional, alternative approaches which a PCB designer may have at his or her disposal. With the trend increasing that more and more PCBs are being designed with substrates that are injection molded out of thermoplastic or thermoset materials--rather than being produced using the traditional glass/epoxy FR-4 lamination process--it would be further desirable to provide an approach for alleviating solder joint cracking which utilizes some of the advantages of injection molding the substrate.